The present invention relates to a high-frequency element comprising a plurality of strip lines, more particularly to a chip-type directional coupler.
Conventional high-frequency elements comprising a plurality of strip lines are illustrated as a directional coupler in FIG. 4. In this directional coupler, a main line and a sub-line are disposed on the same substrate as a planar type. Two strip lines 53, 54 are disposed in parallel at an interval S on a front surface of a dielectric substrate 51 formed with a ground conductor 52 on a rear surface. Each of the parallel portions of the strip lines 53, 54 is as long as one-fourth a wave length of an electromagnetic wave propagating through the dielectric substrate 51.
A high-frequency signal supplied from a port P.sub.1 passes through a strip line 53 (main line) and exits from a port P.sub.2. At this time, by coupling of the strip line 53 (main line) and the strip line 54 (sub-line), part of electric power of the high-frequency signal passing through the strip line 53 flows into the strip line 54 and goes to the port P.sub.3. As a result, there is no output at port P.sub.4. Next, when a high-frequency signal flows through the strip line 53 (main line) in an opposite direction, namely from a port P.sub.2 to a port P.sub.1, part of electric power of the high-frequency signal goes to a port P.sub.4, not to a port P.sub.3. Thus, with this structure, part of an electric signal passing through the strip lines 53 (main line) in a forward direction and part of the signal passing in an opposite direction can be led to the output ports P.sub.3 and P.sub.4, respectively, of the strip line 54 (sub-line). This is the basic operation of the directional coupler.
The coupling of the main line 53 to the sub-line 54 can be controlled by adjusting a distance S between the parallel portions of the two strip lines.
In FIG. 4, illustrating the conventional directional coupler, the main line is a strip line 53 while the sub-line is a strip line 54. However, because of the structural symmetricity, the main line and the sub-line may be interchangeable without affecting the basic operation of the directional coupler.
FIG. 5 shows another conventional directional coupler in which a strip line 65 (main line) and a strip line 66 (sub-line) are stacked. In this example, a strip line 66 is formed on a front surface of a dielectric substrate 61 coated with a ground conductor 64 on a rear surface. Disposed above the substrate 61 is a dielectric substrate 62 formed with a strip line. 65 (main line) such that the strip line 65 (main line) is separated from the strip line 66 (subline) by a distance D. Disposed thereon is a protective dielectric substrate 63.
These three substrates are laminated and sintered, and external electrodes P.sub.1, P.sub.2, P.sub.3, P.sub.4 are attached thereto to complete a directional coupler, which is perspectively shown in FIG. 6.
The directional coupler of FIG. 6 may be operated in the same manner as in FIG. 4. A high-frequency signal supplied from a port P.sub.1 passes through a strip line 65 (main line) and exits from a port P.sub.2. At this time, by coupling of the strip line 65 (main line) and the strip line 66 (sub-line), part of electric power of the signal passing through the strip line 65 flows into the strip line 66 and goes to the port P.sub.3. As a result, there is no output at port P.sub.4. Next, when an electric signal flows through strip line 65 (main line) in an opposite direction, namely from a port P.sub.2 to a port P.sub.1, part of electric power of the signal goes to a port P.sub.4, not to a port P.sub.3. Thus, with this structure, part of electric power of the signal passing through the strip line 65 (main line) in a forward direction and that in an opposite direction can be separated and led to the output ports P.sub.3 and P.sub.4, respectively, of the strip line 66 (sub-line).
The coupling of the main line 65 to the sub-line 66 can be controlled by adjusting a distance in a laminating direction between parallel portions of the two strip lines, namely a thickness D of a dielectric substrate 62.
Such a directional coupler having a function to separate high-frequency signals depending on its direction may be used for controlling output power of microwave signals, for instance, in transmitters of portable telephones. FIG. 7 is a block diagram showing one example of circuits comprising such directional couplers. A directional coupler 71 comprises a main line 72 having ports P.sub.1, P.sub.2 disposed between a transmitted signal-amplifying means (simply amplifier) 101 and an antenna 74; and a sub-line 73 having a port P.sub.3 connected to an automatic gain-controlling circuit 102 and another port P.sub.4 connected to a grounded resistor electrode 75 for absorbing electric power. With such a circuit, part of the output from the amplifier 101 connected to a modulator 103 goes to a port P.sub.3 only, and returns to the automatic gain-controlling circuit 102. Part of the high-frequency signal returning from the antenna 74 goes to a port P.sub.4 and is absorbed by the grounded resistor electrode 75. The output signal from the automatic gain-controlling circuit 102 is supplied to the amplifier 101 having a controllable gain to control high-frequency output in order to maintain the suitable transmission for various circumstances.
However, it is important to miniaturize portable telephones, and the directional couplers used in such portable telephones for the above-mentioned purposes are also required to be made small. In the conventional directional couplers of one-fourth wave length as illustrated in FIG. 4, the strip line electrode should be as long as 2.5 cm (corresponding to 1/4 wave length at a relative dielectric constant .epsilon..sub.r of about 9) at 1 GHz, making it difficult to sufficiently miniaturize the directional couplers. Further, it may be conceivable to use a material having a larger relative dielectric constant in order to shorten one-fourth the wave length, but the strip line should have an extremely small width to keep an impedance of 50 .OMEGA., and the strip line should be arranged at an extremely small distance to obtain a desired coupling of the main line and the sub-line. In order to realize these components, high working precision is required, making it difficult to mass-produce the directional couplers, and also lowering the power capabilities of the directional couplers.
Also, in a structure in which a plurality of strip lines are laminated vertically as shown in FIG. 5, control range may be widened since two strip lines are coupled in a plane. However, as far as miniaturization is concerned, this structure does not work. As a measure for miniaturization, it may be possible to shorten the strip line less than one-fourth the wave length. The characteristics of a directional coupler tentatively manufactured along this idea (shown in FIG. 5) are shown by dotted lines in FIGS. 3A and 3B. Here, a propagation loss from a port P.sub.1 to a port P.sub.2 is called "insertion loss (see FIG. 3A)," a propagation loss from a port P.sub.1 to a port P.sub.3 is called "coupling loss (see FIG. 3B)," and a propagation loss from a port P.sub.1 to a port P.sub.4 is called "isolation." With respect to the directional coupler, the insertion loss should be as small as possible, while the isolation should be as large as possible. The coupling loss is a parameter given by an overall circuit design in a portable telephone, etc.
As is shown by the dotted lines in FIGS. 3A and 3B, when the strip lines are merely made longer in the conventional directional coupler, high isolation cannot sufficiently be achieved in a wide frequency range.